Working with big models requires a good balance between the technical requirements of the model and the technical requirements of the user. Although every virtual model, whether it is 2d, 3d or 4d, may be regarded as a particular form of a general data base, it is clear that is not, at the present time, a very flexible data base. It does not behave like a relational data base that can be inspected in a flexible way. On the contrary, it has a rigid structure, a hierarchical structure that is well suited for performance but is badly suited for navigating through the data and gathering derived information. These are well known disadvantages and advantages, related to the evolution of the data base software that has moved, in the last 30 years, from a hierarchical to a relational structure. These considerations are relevant for any kind of architectural or engineering model. But are particularly pertinent in the case of the model of a city where everything must have its place, and should relate properly with other parts of the model, be susceptible of further modifications and be able to receive new information. These and other related issues have been encountered and developed during the construction of several models at our Laboratory at the ETS Architecture of Barcelona. Our paper explains the main decisions we had to take during the course of these works with special emphasis on those aspects related with the organization of different kind of data in a unified whole that had to be sent to other professionals and had to be, for that reason, organized in a clear and comprehensible way for its further development.